(1) Field of the Invention
The present invention relates to an update method of downloading a program and replacing an existing program with the downloaded program, and more particularly to executing a downloaded program in a digital television by temporarily replacing an existing program with such downloaded program.
(2) Description of the Related Art
Conventional techniques for downloading and updating programs in a digital television are disclosed in Japanese Laid-Open Patent application No. 10-326192 and Japanese Laid-Open Patent application No. 2003-122587. FIG. 1 is a diagram showing the structure of an existing digital cable television system for downloading and executing programs. This system is comprised of a head end 4110 and two terminal apparatuses 4120 and 4130. The head end 4110 stores three programs of a program A4111, a program B4112, and a program C4113, and supplies these programs to the terminal apparatuses 4120 and 4130. The terminal apparatus 4120 is equipped with a processor 4124 for executing three programs which it pre-stores, a program A4131, a program B4132, and a program C4133. Upon the receipt of a new program from the head end 4110, the terminal apparatuses 4120 and 4130 replace the existing programs with such new program, store updated program, and execute it from then on. For example, when the head end 4110 sends the new program A4111 to the terminal apparatuses 4120 and 4130, the terminal apparatuses 4120 and 4130 replace the programs A4121 and 4131 with such program A4111. From then on, the processors 4124 and 4134 execute the updated program A4111 instead of the programs A4121 and 4131. As described above, by updating programs which terminal apparatuses hold, it becomes possible to update their functions as well as to add new functions to such terminal apparatuses.
However, since existing programs are updated according to the existing techniques, it is not easy to restore the programs to the state before they were updated. Thus, when wishing to provide a terminal apparatus with a certain function only for a limited period or length of time, it becomes necessary to download the programs from the head end again so as to restore them. In general, program downloading consumes much time and prevents the user from using another function of the terminal apparatus. Thus, there arises the problem that the user cannot use the terminal apparatus when such user is required to download programs more frequently.